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Date:   Sun, 10 Apr 2022 23:41:57 -0700
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To:     "Kirill A. Shutemov" <kirill@shutemov.name>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Borislav Petkov <bp@alien8.de>,
        Andy Lutomirski <luto@kernel.org>,
        Sean Christopherson <seanjc@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Joerg Roedel <jroedel@suse.de>,
        Ard Biesheuvel <ardb@kernel.org>,
        Andi Kleen <ak@linux.intel.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@linux.intel.com>,
        David Rientjes <rientjes@google.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Ingo Molnar <mingo@redhat.com>,
        Varad Gautam <varad.gautam@suse.com>,
        Dario Faggioli <dfaggioli@suse.com>,
        Brijesh Singh <brijesh.singh@amd.com>,
        Mike Rapoport <rppt@kernel.org>,
        David Hildenbrand <david@redhat.com>, x86@kernel.org,
        linux-mm@kvack.org, linux-coco@lists.linux.dev,
        linux-efi@vger.kernel.org, linux-kernel@vger.kernel.org,
        Mike Rapoport <rppt@linux.ibm.com>
References: <20220405234343.74045-1-kirill.shutemov@linux.intel.com>
 <20220405234343.74045-2-kirill.shutemov@linux.intel.com>
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 <20220409175210.xik3ue3shpagskvi@box.shutemov.name>
From:   Dave Hansen <dave.hansen@intel.com>
Subject: Re: [PATCHv4 1/8] mm: Add support for unaccepted memory
In-Reply-To: <20220409175210.xik3ue3shpagskvi@box.shutemov.name>
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On 4/9/22 10:52, Kirill A. Shutemov wrote:
> On Fri, Apr 08, 2022 at 12:11:58PM -0700, Dave Hansen wrote:
>> On 4/5/22 16:43, Kirill A. Shutemov wrote:
>>> Kernel only needs to accept memory once after boot, so during the boot
>>> and warm up phase there will be a lot of memory acceptance. After things
>>> are settled down the only price of the feature if couple of checks for
>>> PageUnaccepted() in allocate and free paths. The check refers a hot
>>> variable (that also encodes PageBuddy()), so it is cheap and not visible
>>> on profiles.
>>
>> Let's also not sugar-coat this.  Page acceptance is hideously slow.
>> It's agonizingly slow.  To boot, it's done holding a global spinlock
>> with interrupts disabled (see patch 6/8).  At the very, very least, each
>> acceptance operation involves a couple of what are effectively ring
>> transitions, a 2MB memset(), and a bunch of cache flushing.
>>
>> The system is going to be downright unusable during this time, right?
...
>> Do we need anything more discrete to tell users when acceptance is over?
> 
> I can imagine setups that where acceptance is never over. A VM running
> a workload with fixed dataset can have planty of memory unaccepted.
> 
> I don't think "make it over" should be the goal.

I agree, there will be users that don't care when acceptance is over.
But, I'm also sure that there are users that will care deeply.

>>  For instance, maybe they run something and it goes really slow, they
>> watch "accept_memory" until it stops.  They rejoice at their good
>> fortune!  Then, memory allocation starts falling over to a new node and
>> the agony beings anew.
>>
>> I can think of dealing with this in two ways:
>>
>> 	cat /sys/.../unaccepted_pages_left
>>
>> which just walks the bitmap and counts the amount of pages remaining. or
>> something like:
>>
>> 	echo 1 > /sys/devices/system/node/node0/make_the_pain_stop
>>
>> Which will, well, make the pain stop on node0.
> 
> Sure we can add handles. But API is hard. Maybe we should wait and see
> what is actually needed. (Yes, I'm lazy.:)

Let's just call out the possible (probable?) need for new ABI here.
Maybe it will cue folks who care to speak up.